1. Field of the Invention
The present invention relates to a method and apparatus for handling a timer for triggering a buffer status report (BSR), and more particularly, to a method and apparatus for handling a timer for triggering a BSR in a user equipment (UE) of a wireless communication system, so as to avoid waste of network resources.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in base stations (Node Bs) alone rather than in Node Bs and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE system, based on 3GPP specification “TS 36.321 V8.3.0,” a Buffer Status Report (BSR) is used to provide the network with information about the amount of data in uplink (UL) buffers of a UE. The BSR will generate a BSR MAC Control Element, which includes information about the amount of data available for transmission in RLC layer and Packet Data Convergence Protocol (PDCP) layer, when being triggered. The BSR shall be triggered when UL data becomes available for transmission and the data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, in which case the BSR is referred to as “Regular BSR.” When the BSR is triggered but there is no allocated resource for new transmission, the UE would then trigger a Scheduling Request (SR) procedure for requesting uplink resources, i.e. UL-SCH resources, to send the BSR MAC Control Element. Note that only the Regular BSR can trigger the SR procedure when the UE has no UL resources allocated for new transmission for a current transmission time interval (TTI).
When the BSR MAC Control Element is transmitted, the network may not be able to successfully receive the BSR MAC Control Element, and thus would not allocate any uplink transmission resources to the UE. In this case, if the reason for triggering the BSR is no longer satisfied, such as no higher priority data becomes available for transmission, for example, the UE would have no uplink transmission resources for use and enter into a deadlock situation. Therefore, in order to solve this problem, the prior art proposes a BSR retransmission mechanism, which utilizes a Retransmission BSR Timer, i.e. a timer RETX_BSR_TIMER, to enhance the reliability of BSR transmission. The UE starts the timer RETX_BSR_TIMER when a BSR MAC control element is generated, and restarts (if running) the timer RETX_BSR_TIMER when UL resources allocated for new transmission are received, e.g. on the PDCCH or in a Random Access Response, which indicates the BSR MAC Control Element is successfully received by the network. When the timer RETX_BSR_TIMER expires and the UE has data available for transmission in the buffer, the UE shall trigger a BSR, in which case the BSR is also referred to as “Regular BSR.”
However, the LTE system has two scheduling methods: dynamic scheduling (DS) and semi-persistent scheduling (SPS). For DS, the network dynamically allocates resources to UEs for data reception or transmission depending on traffic volume, quality of service (QoS) requirements of each UE. And for SPS, the network periodically allocates resource to UEs, in order to serve upper layer applications which generate semi-static size data periodically, e.g. Voice over Internet Protocol (VoIP) services, for reducing control information sent on a physical downlink control channel (PDCCH) and enhancing system scheduling performance. In other words, SPS provides semi-persistent transmission resources, i.e. configured UL grant, to the UE, such that the UE can perform periodic data transmission without receiving PDCCH.
Please refer to FIG. 1, which illustrates operation of the timer RETX_BSR_TIMER when SPS is activated. As mentioned above, semi-persistent transmission resources would be allocated to the UE by SPS at a fixed period, such as at timing point A, B, C and E. Assume that the UE sends a BSR MAC Control Element to the network at a timing point A and starts the timer RETX_BSR_TIMER. Under a situation that the SPS UL grant has been configured, the network may not allocate dynamic UL resources to the UE because the UE already has configured UL grant to do UL transmission. However, according to the operation of the timer RETX_BSR_TIMER, if no UL resource for new transmission has been received upon expiration of the timer RETX_BSR_TIMER, such as at timing point D, the UE shall trigger a BSR, and thus further triggers the SR procedure. In this case, the BSR and SR are triggered unnecessarily, which causes waste of network resources.